Bringing extruded plane to one level
I presume, you want to flatten the mesh, to do this select all the faces and press S + Y + 0.  

This should give you the flat plane you are looking for.

In future when modelling, you can replace "Y" with any axis and also when extruding planes, be sure that in the side view the face/edge is flat so that you can extrude worry free in the appropriate orthographic view.